As radiation generating apparatuses have reduced in size and the energy of radiation emitted has increased, enhancing the withstand voltage characteristics (hereinafter referred to as withstand voltage) of radiation generating tubes has been required. One of portions of radiation generating tubes that require withstand voltage is a bonded portion of an insulating tubular member and a cathode. Particularly when the bonded portion is exposed from the radiation generating tube as viewed from the outside thereof, electric field concentration tends to occur in the vicinity of the exposed portion, where discharge is prone to occur. PTL 1 discloses providing a corona ring so as to cover a fusing portion of a glass vacuum envelope and a cathode to relax electric field concentration in the vicinity of the fusing portion, thereby preventing damage to the vacuum envelope due to a local discharge impact.
PTL 2 discloses a structure in which an electrically conducting portion located at the side of a radiation emission window and an electrically insulating portion located at the side of a voltage apply portion are fixed, and an electrically resistive film is disposed outside the electrically insulating portion to prevent discharge due to the disturbance of the electric field between a corona ring and a vacuum envelope.